— A good localization ability is essential for an autonomous vehicle to perform any functions. For ground vehicles operating in outdoor, uneven and unstructured environments, the localization task becomes much more difficult than in indoor environments. In urban or forest environments where high buildings or tall trees exist, GPS sensors also fail easily. The main contribution of this paper is that a multiaided inertial based localization system has been developed to solve the outdoor localization problem. The multi-aiding information is from odometry, an accurate gyroscope and vehicle constraints. Contrary to previous work, a kinematic model is developed to estimate the inertial sensor’s lateral velocity. This is particularly important when cornering at speed, and side slip occurs. Experimental results are presented of this system which is able to provide a vehicle’s position, velocity and attitude estimation accurately, even when the testing vehicle runs in outdoor uneven envi...